Caption: MODEL RELEASED. Water treatment. Operator opening an external valve on an upward flow clarifier tank at a water treatment works. The water inside the tank has been treated with a coagulant. As the water is forced to flow upwards, the particles within it bind together in a sludge. The sludge is skimmed off and treated. The water from this stage is sent to the rapid gravity filters. For an image of the rapid gravity filters, see T825/225. Photographed at the River Itchen Treatment Works, Gaters Mill, Southampton, UK.